12.1 Chapter 12 Multiple Access Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12.2 Figure 12.1 Data link layer divided into two functionality-oriented sublayers 12.3 Figure 12.2 Taxonomy of multiple-access protocols discussed in this chapter 12.4 12-1 RANDOM ACCESS 12-1 RANDOM ACCESS In In random access random access or or contention contention methods, no station is methods, no station is superior to another station and none is assigned the superior to another station and none is assigned the control over another. No station permits, or does not control over another. No station permits, or does not permit, another station to send. At each instance, a permit, another station to send. At each instance, a station that has data to send uses a procedure defined station that has data to send uses a procedure defined by the protocol to make a decision on whether or not to by the protocol to make a decision on whether or not to send. send. ALOHA Carrier Sense Multiple Access Carrier Sense Multiple Access with Collision Detection Carrier Sense Multiple Access with Collision Avoidance Topics discussed in this section: Topics discussed in this section: 12.5 Figure 12.3 Frames in a pure ALOHA network 12.6 Figure 12.4 Procedure for pure ALOHA protocol 12.7 The stations on a wireless ALOHA network are a maximum of 600 km apart. If we assume that signals propagate at 3 × 10 8 m/s, we find T p = (600 × 10 5 ) / (3 × 10 8 ) = 2 ms. Now we can find the value of T B for different values of K . a. For K = 1, the range is {0, 1}. The station needs to| generate a random number with a value of 0 or 1. This means that T B is either 0 ms (0 × 2) or 2 ms (1 × 2), based on the outcome of the random variable. Example 12.1 12.8 b. For K = 2, the range is {0, 1, 2, 3}. This means that T B can be 0, 2, 4, or 6 ms, based on the outcome of the random variable. c. For K = 3, the range is {0, 1, 2, 3, 4, 5, 6, 7}. This means that T B can be 0, 2, 4, . . . , 14 ms, based on the outcome of the random variable. d. We need to mention that if K > 10, it is normally set to 10. Example 12.1 (continued) 12.9 Figure 12.5 Vulnerable time for pure ALOHA protocol 12.10 A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps. What is the requirement to make this frame collision-free? Example 12.2 Solution Average frame transmission time T fr is 200 bits/200 kbps or 1 ms. The vulnerable time is 2 × 1 ms = 2 ms. This means no station should send later than 1 ms before this station starts transmission and no station should start sending during the one 1-ms period that this station is sending. [...]... of 250 will probably survive 12. 18 Figure 12. 8 Space/time model of the collision in CSMA 12. 19 Figure 12. 9 Vulnerable time in CSMA 12. 20 Figure 12. 10 Behavior of three persistence methods 12. 21 Figure 12. 11 Flow diagram for three persistence methods 12. 22 Figure 12. 12 Collision of the first bit in CSMA/CD 12. 23 Figure 12. 13 Collision and abortion in CSMA/CD 12. 24 Example 12. 5 A network using CSMA/CD... idle 12. 30 Figure 12. 17 Flow diagram for CSMA/CA 12. 31 12- 2 CONTROLLED ACCESS In controlled access, the stations consult one another to find which station has the right to send A station cannot send unless it has been authorized by other stations We discuss three popular controlled -access methods Topics discussed in this section: Reservation Polling Token Passing 12. 32 Figure 12. 18 Reservation access. .. in this section: Reservation Polling Token Passing 12. 32 Figure 12. 18 Reservation access method 12. 33 Figure 12. 19 Select and poll functions in polling access method 12. 34 Figure 12. 20 Logical ring and physical topology in token-passing access method 12. 35 12- 3 CHANNELIZATION Channelization is a multiple- access method in which the available bandwidth of a link is shared in time, frequency, or through... frame is 10 Mbps × 51.2 μs = 512 bits or 64 bytes This is actually the minimum size of the frame for Standard Ethernet 12. 25 Figure 12. 14 Flow diagram for the CSMA/CD 12. 26 Figure 12. 15 Energy level during transmission, idleness, or collision 12. 27 Figure 12. 16 Timing in CSMA/CA 12. 28 Note In CSMA/CA, the IFS can also be used to define the priority of a station or a frame 12. 29 Note In CSMA/CA, if the... the throughput is 250 × 0.152 = 38 Only 38 frames out of 250 will probably survive 12. 13 Figure 12. 6 Frames in a slotted ALOHA network 12. 14 Note The throughput for slotted ALOHA is S = G × e−G The maximum throughput Smax = 0.368 when G = 1 12. 15 Figure 12. 7 Vulnerable time for slotted ALOHA protocol 12. 16 Example 12. 4 A slotted ALOHA network transmits 200-bit frames on a shared channel of 200 kbps... through code, between different stations In this section, we discuss three channelization protocols Topics discussed in this section: Frequency-Division Multiple Access (FDMA) Time-Division Multiple Access (TDMA) Code-Division Multiple Access (CDMA) 12. 36 ... per millisecond The load is 1 In this case S = G× e−2 G or S = 0.135 (13.5 percent) This means that the throughput is 1000 × 0.135 = 135 frames Only 135 frames out of 1000 will probably survive 12. 12 Example 12. 3 (continued) b If the system creates 500 frames per second, this is (1/2) frame per millisecond The load is (1/2) In this case S = G × e −2G or S = 0.184 (18.4 percent) This means that the throughput... per millisecond The load is 1 In this case S = G× e−G or S = 0.368 (36.8 percent) This means that the throughput is 1000 × 0.0368 = 368 frames Only 386 frames out of 1000 will probably survive 12. 17 Example 12. 4 (continued) b If the system creates 500 frames per second, this is (1/2) frame per millisecond The load is (1/2) In this case S = G × e−G or S = 0.303 (30.3 percent) This means that the throughput...Note The throughput for pure ALOHA is S = G × e −2G The maximum throughput Smax = 0.184 when G= (1/2) 12. 11 Example 12. 3 A pure ALOHA network transmits 200-bit frames on a shared channel of 200 kbps What is the throughput if the system (all stations together) produces a 1000 frames per second b 500 frames per . sublayers 12. 3 Figure 12. 2 Taxonomy of multiple- access protocols discussed in this chapter 12. 4 12- 1 RANDOM ACCESS 12- 1 RANDOM ACCESS In In random access random. 12. 1 Chapter 12 Multiple Access Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 12. 2 Figure 12. 1 Data